I haven't seen the entire video, but the same day they announced that Snake wouldn't keep his role, I came up with a theory. The protagonist in the next Metal Gear Solid game is someone who believes that he is Snake due to a traumatic event, drugs etc. Which is why they switched the voice actor in order to confuse the player by adding a psychological feeling in the game.
Now I haven't played the game (except for Metal Gear Rising), but as far as I know there aren't any supernatural beings in the series. When we saw the Phantom Pain trailer it took place on a hospital and it appears that supernatural beings appears (hallucinations?), which may support this theory.